DESCRIPTION (Investigator's Abstract): Extracellular proteases have been implicated in various functions in the mammalian central nervous system (CNS). However, it has been difficult to analyze these possible functions in vivo. Using an experimental model to induce neuronal damage in the mouse hippocampus, we have shown that mice deficient for the serine protease tPA (tPA-/-) are strikingly resistant to neuronal degeneration. Mice deficient for plasminogen, the classical tPA substrate, exhibit the same resistant phenotype as the tPA-/- mice. Both tPA and plasminogen are synthesized in the hippocampus, and the expression of proteolytic activity appears to be modulated by endogenous inhibitors. Finally, infusion of tPA/plasmin inhibitors into the hippocampus of wild-type mice can also confer resistance to neuronal death. A central hypothesis emerges from these results and from work in other laboratories: An extracellular proteolytic cascade of tPA and plasmin mediates neuronal cell death in the mammalian CNS. In this application, we propose to investigate the mechanism by which tPA/plasmin function in the CNS by addressing the following questions: 1) What structural and catalytic characteristics of tPA and plasmin are critical for degeneration? Can protease inhibitors be identified that participate in regulating neuronal death, and that have therapeutic potential in retarding degeneration? 2) Are there endogenous protease inhibitors that help regulate tPA and plasmin activity? 3) What are the respective roles of tPA produced by neurons and microglia? 4) What are the specific substrates whose cleavage by tPA and/or plasmin mediates neuronal degeneration? Answering these questions will help define the mechanism by which tPA and plasminogen function in the hippocampus, and could have implications for the treatment of many neurodegenerative disorders.